Cathode ray tube exposure optics



May 5, 1970 cs. A. BURDICK CATHODE RAY TUBE EXPOSURE OPTICS 7 Sheets- Sheet 1 Filed Dec. 29, 1966 TUBE AX/S K c m w. N JN warm M M N A u 1 Ga May 5, 1970 G. A. BURDICK 3,509,802

CATHQDE RAY TUBE EXPOSURE OPTICS Filed Dec. 29, 1966 7 Sheets-Sheet 2 INVENTOR. 64 EN A. Bu/w/cK .KKMJJW 7 Arm/war G. A.=BURDICK CATHODE RAY TUBE EXPOSURE OPTICS May 5, 1970 7 Sheets- Sheet 3 Fil ed Dec. 29, 1966 INVENTOR. GLEN ABuRo/cK A 7' TOR/V5 Y May 5, 1970 G. A. BURDICK CATHODE RAY TUBE EXPOSURE OPTICS Filed Dec. 29, 1966 7 Sheets- Sheet 4 .I'NVENTOR. G'mv 4. BURD/CK n M. M

ATTORNEY G. A. BURDICK CATHODE RAY TUBE EXPOSURE OPTICS May 5, 1970 7 Sheets- Sheet 5 Filed Dec. 29, 1966 H M 2 7 g 9 x w I 1 IWHMYI J 7 3 W 9 2 I. l F 9 2 m? w, a, offim ml d/ W ll! 9 r I. 4 4 5y K mm H M N R EU R VB 0 ..N. 7 Z Y B y 5, 1970 G. BURDICK 3,509,802

CATHODE RAY TUBE EXPOSURE OPTICS Filed Dec. 29, 1966 7 Sheets-Sheet a 99 89 ,89 N "7 7 HT E .95! m Y N gg i H Ti in INVENTOR. GLEN 14. Bum/ck ATTORNEY y 5, 1970 G. A. BURDICK 3,509,802

CATHODE RAY TUBE EXPOSURE OPTICS Filed Dec. 29, 1966 7 Sheets-Sheet '7 HTHWTII INVENTOR. GLEN ABURD/cK ATTORNEY United States Patent ABSTRACT OF THE DISCLOSURE An optical system utilized for providing a tri-dot patterned color cathode ray tube screen having phosphor patterns of improved symmetry. The optical system is predeterminately positioned to expose a sensitized screen ,panel through an associated apertured mask. A portion of the exposure light radiation is refracted by a modified plano-concave lens having at least one modified plano chordal portion whereby improved exposure is provided for one or more specific peripheral quadrantal portions of the screen to effect substantially improved equilateral phosphor dot pattern formation thereon.

This invention relates to cathode ray tubes and more particularly to improved patterned screens and a method and optical apparatus utilized in forming said improved screens for color cathode ray tubes.

Cathode ray tubes employed as display devices in color television applications conventionally utilize one or more electron guns to provide the required electron beam or beams which are accelerated, focused and modulated by voltages applied to the gun. When a plurality of guns are used in a related manner, convergence electrodes or pole pieces are usually included as part of the electron gun structure. The modulated electron beams are discretely deflected across the screen to provide electron impingement upon selected color fiuorescing materials disposed in patterned configurations on the viewing panel of the tube to reproduce the transmitted color image. It is conventional practice to position a grid or grids or an apertured mask intermediate the electron gun or guns and the cathodoluminescent screen to provide focusing and deflection of the electron beams or selected masking of the screen.

Color cathode ray tubes, of the type generally utilized for color television, usually have screens consisting of multitudinous dot, bar or stripe patterned formations of green, blue, and red color fiuorescing phosphors. Various methods have been employed to form the respective screens, for example, one conventional process makes use of a photoprinting technique. By this procedure, the tube viewing panel is coated with a light sensitive substance and one of the desired electron-responsive phosphor materials and then exposed to a point source of light through an appropriate negative master. Subsequent .development produces a first phosphor pattern of desired configurations. The process is repeated for each of the remaining different color electron-responsive phosphors comprising the patterned screen. In each instance, the point source of light is appropriately offset during the exposure operation to provide individual phosphor patterns that are properly displaced from one another to form the desired screen.

One difliculty encountered with photo-disposed screens is caused by the electrons not following the same beampaths during tube operation as the light rays travel during the screen forming procedures. Consequently, there are areas of the screen where the electron beams do not properly impinge the desired patterned configurations during tube operation. This condition of misregister results in the display image having color impurity.

3,509,802 Patented May 5, 1970 The inherent nature of the electron beam is a factor affecting the aforementioned misregister. Since the electrons projected toward the screen have mass and charge, their paths of travel are altered by the electron gun and tube geometry and by various electrostatic and magnetic fields existing in and around the tube. It has been found that the center of deflection, or the location within the deflection yoke from which the electrons appear to come, moves as the electron beam scans the screen. In addition, when several electron guns are utilized to effect a plurality of electron beams the phenomena resulting from the dynamic convergence of the several beams causes a departure of the beams from their normal paths of travel.

Various procedures and devices for reducing the amount of misregister between the electron beam or beams and the fluorescent pattern configurations have been employed. Auxiliary devices have been used internally and externally of the tube and on and about the tube components in an effort to compensate for the excursion of the electron beams from the desired trajectories. In the screen exposure devices, several light optical systems have been devised to consummate the photodeposition of the desired screen pattern. For example, in tubes having a discretely dotted screen pattern formed relative to a foraminous shadow mask wherein a triad of dots of differing cathodoluminescent phosphors are related to each aperture in the mask, several types of lens components have been incorporated into the light optical system. For instance, in color tubes having a 70 degree angle of deflection, it has been found that use of a spherical symmetrical planar-concave lens, properly tilted and offset, provides light optics which substantially duplicate the electron optics of the operating tube as effected by the axial motion of the center of deflection and dynamic convergence. Screen exposure systems of this type are described and claimed in Pat. No. 2,986,080 issued to Glen A. Burdick, and Pat. No. 2,936,683 issued to Glen A. Burdick et al., both of which are assigned to the same assignee as the present invention.

In the manufacture of screens for color tubes having angles of deflection greater than 70 degrees, for instance, rectangular tubes having degree deflection, it has been discovered that use of the aforementioned lens does not achieve the desired register relationship between light optics and electron optics. In at least two areas of the screen, in substantially the upper right and upper left hand corner areas, the desired equilateral dot patterns are not fully realized. A related lack of symmetry exists in the 70 degree tube exposure optics, but due to the smaller angle of deflection the effect is not significant in magnitude, and the dot pattern-beam registration is acceptable for all areas of the screen. For the most part, in the screen of the 90 degree tubes, the dot and beam patterns are substantially in desired registry for all parts of the screen except in the areas of the two mentioned upper corner regions of the viewed screen. In the upper right corner areas the dots of one color tend to be displaced in an outward radial direction in their triad patterns, and in the upper left corner area dots of another color are affected in a similar manner.

Accordingly, it is an object of the invention to reduce the aforementioned difficulties and to provide an improved color cathode ray tube.

Another object is to provide a color cathode ray tube screen having a dot pattern of improved symmetry.

Still another object is to provide an improved shadow mask color cathode ray tube having improved register of electron beam impingement on the phosphor dot pattern.

A further object is to provide an improved method and optical system for producing color screens having dot patterns of improved symmetry.

The foregoing objects are achieved in one aspect of the invention by the provision of an improved method and an associated optical system for use in a light exposure device for photo-fabricating an improved color cathode ray tube screen having a patterned mask in spaced adjacency there with. The light radiation for screen exposure is discretely refracted by a modified plano-concave lens having nonsymmetrical optical aberration positioned between a light source and the masked sensitized panel. Improved light refraction is achieved by utilizing the modified lens in a manner that the modified portion of the plano surface formed as at least one substantially chordal area is oriented to provide desired correction of light ray refraction and direction to a discrete area of the screen to thereupon effect the optical imprinting of a screen pattern having improved symmetry.

For a better understanding of the present invention, together with other and further objects, advantages, and capabilities thereof, reference is made to the following specification and appended claims in connection with the accompanying drawings in which:

FIG. 1 is a cross sectional view of a color cathode ray tube of the shadow mask type employing plural electron beams;

FIG. 2 is a section view showing the manner in which the electron beams are converged in the tube;

FIG. 3 is a partial plan view illustrating a prior art rectangular screen wherein the dots and beam landings are exaggerated in size;

FIG. 4 is a section plan view of a type of apparatus utilized in photo-disposing patterned cathode ray tube screens taken along the line 44 of FIG. 5;

FIG. 5 is a top plan view looking down through the panel into the apparatus;

FIG. 6 is a plan view showing the optically utilized portion of the refractive medium;

FIG. 7 is a sectional view of the refractive medium taken along the line 7-7 of FIG. 6;

FIG. 8 is a sectional view showing another embodiment of the invention;

FIG. 9 is a sectional view showing the transverse profile of the modified first chordal portion taken along the line 99 of FIG. 7;

FIGS. 10 and 11 are sectional views showing alternate embodiments of the transverse profile of the modified first chordal portion taken along the line 99 of FIG. 7;

FIG. 12 is a sectional view showing the transverse profile of the modified section chordal portion taken along the line 1212 of FIG. 8; and

FIGS. 13 and 14 are sectional views showing alternate embodiments of the transverse profile of the modified second chordal portion taken along the line 12-42 of FIG. 8.

With reference to the drawings, FIG. 1 illustrates a conventional plural beam shadow mask type of color cathode ray tube 11 having a central axis 12 therethrough. Suitably disposed within the neck portion of the envelope 13 are three electron emitters 15 oriented, for example, substantially 120 degrees apart equally spaced about axis 12 to provide a delta arrangement of electron beams 17, 18 and 19. Coil means in the form of yoke 20 positioned external of the envelope, are utilized to deflect the electron beams over the raster area. It is desirable that the several beams converge at the mask 21 and pass through apertures 22 therein to discretely impinge the patterned cathodo-luminescent screen 23 therebeneath. The screen comprises a multitude of triadically arranged dot or elements of green, blue and red color electron responsive fiuorescing materials formed on the interior surface of the viewing panel 25. While a tri-gun shadow mask color tube is illustrated in FIG. 1, it is not intended to be limiting as the invention to be described in this specification is also applicable in other types of image reproduction devices employing plural beams of radiant energy excitation.

Since the tube axis, the panel axis and the electron gun system axis are substantially coincidental, it seems expedient for clarification in this description to henceforth denote these several reference axes as the central axis 12.

To achieve the desired convergence or crossover of the several beams at the mask apertures, external dynamic convergence means 27 are normally utilized. With reference to FIG. 2, two of the three beams, 17 and 18, respectively, are shown to illustrate the aspects of dynamic convergence. The static convergence beam paths, 17 and 18, passing through points a and b, respectively, in the deflection region, proceed at an angle with the tube axis to converge or crossover at the apertured mask 21 and thence impinge the patterned screen 23. In deflecting the beams to an angle alpha (a) without the influence of the dynamic convergence means 27, the beams appear to come from points a and b and undesirably effect convergence short of the mask at point Employment of the dynamic convergence means provides magnetic fields which move the beam positions 17 and 18 radially outward in the deflection region to cause the beam positions 17 and 18 to appear to come from points c and d to provide the desired convergence crossover at the apertured mask. It will be observed that as the angle of deflection increases, the deflected electron beam appears to emerge from the deflection region at a point closer to the screen. For example, as the electron beam path 17 is deflected from the static position to the convergence deflected beam path 17, the dotted line between a and c defines the locus of motion of the apparent center of deflection. In like manner, movement of the related beam from static beam path 18 to the convergence deflected path 18' defines the locus of motion of the apparent center of deflection as being along the dotted line between b and d. The apparent center of deflection and locus of motion thereof is different for each electron beam because of the respective electron gun orientation within the tube.

Screen 23 is comprised of multitudinous triadical groupings of electron responsive phosphor dots 35, 36, and 37, respectively. Two of such triadical groupings are illustrated in FIG. 2, namely, an axial grouping 31 and a radial grouping 33. The geometry of the tube is such that the three electron beam landings form a substantially equilateral triadical formation at the center of the screen and a radially compressed triadical formation at the peripheral region thereof. With the optical system conventionally utilized for screen exposure, the photo-disposed triadical phosphor dot patterns, while substantially equilateral over a major portion of the screen area, depart significantly from equilaterality in certain peripheral areas. This departure is particularly noticeable in tubes having wide angles of deflection in excess of 70 degrees, as for example degree deflection in rectangular screen tubes. Such is shown in FIG. 3 wherein fragmentary portion of a prior art rectangular screen 39 is portrayed from the viewpoint of an observer facing the viewing panel 23. Illustrative groupings of dots and beam landings are shown in exaggerated size. The axial dot grouping 31 has substantially equilateral placement of phosphor dots 35, 36, and 37. When the electron beams 17, 18, and 19 pass through the mask and make landings on the electron-responsive dots, the areas of impingement fluoresce in a color characteristic of the particular phosphor, such as green (G), red (R), and blue (B), respectively. In the upper left screen region 41, the illustrated radial grouping 33 shows an outward radial displacement of the phosphor dot 45 with reference to adjacent dots 46 and 47. The criticalness of this displacement is evidenced when the deflected beam 17' makes a peripheral G landing there-v on. A similar situation erists in the upper right screen region 51 where an exemplary upper right radial triad 43 has the red phosphor dot 56 radially displaced.

It has been found that correction of the misplaced dots in the aforementioned corner regions of rectangular screens can be achieved by utilizing the method and the specialized optical system of the invention during screen exposure.

An optical exposure apparatus, such as that depicted in FIG. 4, is utilized in the method to form the aforementioned dot patterned screen. It is desired to directly dispose the triadical dot patterns in a manner that the subsequent electron beam landings will be in register therewith and have the largest possible minimum border of fluorescent material around each beam impinging position. Prior to the exposure of each of the several patterns comprising the screen, the screen-bearing surface, in this instance the inner surface of the viewing panel 25, is coated with a light hardenable photosensitive substance and a desired electron responsive color cathodoluminescent phosphor material, one for example being zinccadmium sulfide which fluoresces green, to form a photosensitive phosphor-associated film 61 thereover. Next, the apertured mask is temporarily ositioned in spaced adjacency with the sensitized panel, whereupon the mated mask-panel assembly is suitably oriented on the exposure apparatus 65. Within this apparatus, there are means 67 for predeterminately positioning an optical system 68 comprising a point light source 6 9 and a light refractive medium in the form of a modified plano-concave lens 71 wherein a portion of the plano surface is distinctly modified. In the exposure step, discrete areas of the coated panel are subjected to light radiating from the point light source 69 which is refracted in a predetermined manner by the lens 71 and directed through the mask apertures 22. The discrete areas of the photosensitive film 61 which are exposed to the light radiation become hardened and adhere to the surface of the glass panel forming an imprint of a first screen pattern of dots. sequentially, a screen developing step removes the intervening unexposed portions of the film shadowed by the solid portions of the mask structure wherein the panel is treated with a suitable solvent or developing fluid. The above-described procedure is twice repeated to dispose the required blue and red phosphor dot patterns of the complete screen combination. For the separate exposure of each screen pattern, the light source and lens are properly positioned and offset from the central axis, the optical system being shifted substantially 120 degrees about the central axis for each subsequent pattern exposure.

With reference to FIG. 4, 5, 6, and 7 the essentials of the optical system 68 are shown in greater detail wherein a light refractive medium such as a modified plano-concave lens 71 of high UV transmissive optical glass is positioned intermediate the point light source 69 and the apertured shadow mask 21. As previously described, the locus of motion of the apparent center of deflection in the operating tube appears to move forward toward the screen as the angle of deflection increases. In a like manner the apparent origin of the light source appears to follow a similar locus due to the optical aberrations designed into the lens. Of the light emanating from source 69, a ray 73 designated as a single line is selected for illustration. Upon being incident upon and refracted by lens 71, subject ray is directed to a particular portion of the mask 21 to pass through an aperture therein and light expose a phosphor dot area 75 of the screen. The electron beam for the same deflection angle appears to originate at point M, which also appears to be the apparent light source for ray 73. Another light ray 77, directed to an opposite portion of the screen to expose phosphordot area 79 is likewise refracted to have an apparent light source at M. With N designating the apparent light source for a ray 81 beamed to the center dot area 83, the apparent locus 86 of motion of the light source is along the line M-N. Although the light rays 73 and 77 originate at the point source 69, they appear to come from a point on locus 86, when viewed from the screen, due to refraction of the modified lens 71. Thus (with reference to FIG. 2 and 4) the optical system of the exposure device as utilized in the method of the invention produces the desired special relationship between the respective local of motion and 86 of the apparent center of electronic beam deflection and the loci of the apparent origins of the light beams to effect the desired register between the prosphor dot screen pattern and respective electron beam impingements thereon. The optical system illustrated is, for example, capable of photo-disposing the green fluorescing screen pattern whereof a dot in substantially the upper left corner of 10 oclock area of the viewed screen is designated as 75 and one at suzstantially the lower right corner or 4 oclock area thereof as 79. The relationship of subsequent electron beam impingement on these respective phosphor dots is indicated in FIG. 5 as 76 and 7 6'.

The design of the modified plano-concave lens and the positioning of the lens relative to the masked panel, the central axis and the light source may be adequately achieved in several ways: by mathematical calculation, by empirical experimentation, or by a judicious combination of the two.

The modified plano-concave lens 71 as utilized in the improved optical system of the invention for photo-disposing the screen of a 25 inch rectangular shadow mask tube having substantially 90 degree deflection, has corrective qualities for optical aberration substantially equivalent to the length of the locus of motion of the apparent center of deflection for'all angles included within the subject 90 degree deflection. By way of illustration, the basic circular lens 70 has a symmetrical concave portion or spherical concavity 89 having a radius of curvature 84 of approximately 83.500 inches determined from a point 87 on the basic lens axis 91 which is also referred to as the quasiaxis of symmetry. The thickness of the lens p at this quasiaxis is approximately 0.200 inch. In this example the lens plano portion 93 has a modified chordal section 95 shaped as a substantially circular cylindrical section with the surface 97 thereof transitionally tangent to the plano surface along a substantially linear region of demarcation 96. The axis 98 of the substantially cylindrical section is spacedly oriented from the concave surface 89 in a plane 100 substantially parallel to the plano lens surface 93, the axis 98 of the cylindrical section being substantially parallel to the substantially linear region of demarcation 96 and in a plane 102 therewith perpendicular to the plano surface 93. The radius 94 of the substantially cylindrical surface, as determined from the axis 98, is in the order of 85.00 inches. Thus, a slight convex curvature is imparted to the chordal area in a direction toward the spherical concavity which effects a gradual reduction in the refractive thickness of the lens. It has been found that this chordal modification improved the refraction of the light rays directed to the upper left region of the screen and brings the ray landings radially inward for desirably disposing the green phospher dots.

It has been found that the orientation of the optical system to produce the desired aforedescribed dot placement in the 25 inch rectangular panel can be accomplished by orienting the several axes of related elements of the system in a common vertical plane. For example, there is contained in subject common plane the central axis 12 wherefrom the axis 72 of the point light source 69 is laterally offset by a distance k of approximately 0.180 inch; and wherein there is also located the quasi-axis 91 of the lens component 71 which is offset from the central axis by a distance p of approximately 1.938 inches. Optimizing of the angles of incidence and refraction is effected by imparting a slight tilt to the lens which is achieved by tilting the quasi-axis thereof at an angle (5) of substantially .5 degree from a line 14 parallel with the central axis 12 and oriented in the common vertical plane therewith. Consummation of the lens tilt is accomplished about a lateral axis of placement 92 which perpendicularly intersects the quasi-axis 91 and the lens plane of symmetry 99. The amount of offset and tilt of the modified lens 71 are inter-related in a compensating manner whereof an increase in offset will permit a reduction in tilt-and vice-versa to achieve the desired results.

It will be noted that the basic lens 70 is offset in the exposure device in a manner that only a portion of the spherical concavity 89, removed or shifted eccentrically from said quasi axis, is optically utilized as a non-symmetrical spherical concavity. The concavity in conjunc tion with a portion of the plano surface 93 and the chordal area 95 constitute the elements of utilized light refractive medium 71. In addition, it will be noted that the quasiaxis is retained therein. The portion of the basic lens 70 designated by s is not optically utilized in the application, and if desired can be removed by forming a smaller optical unit therefrom embodied by the dimension t in the form of refractive medium 71' which contains the optical essentials for the application previously designated by refractive portion 71 of basic lens 70. Henceforth, in this description, to enhance clarity, the utilized refractive medium will continue to be referred to as 71 and the extraneous portions of basic lens structure 70 will be disregarded. Due to the rectangular screen shape, differing portions of the refractive medium 71 are utilized to expose the dot pattern of each respective color producing phosphor.

With particular reference to FIG. 5, looking into the panel 25 positioned atop exposure apparatus 65, positioning planes utilized in orienting the optical system for photodisposing the several color fluorescing phosphor patterns are indicated. For example, the green dot pattern is formed by positioning the optical system in plane CD which substantially corresponds to the 4-10 oclock panel diagonal, being removed clockwise from the 12 oclock position in ordinate plane AB by 120 degrees. The plane of symmetry 99 of refractive medium 71 is positioned coincident with plane CD. This plane of lens symmetry bisects the first chordal portion 95 in a manner perpendicular to said substantially cylindrical section and has the quasi-axis 91, the light source axis 72, and the central axis 12 contained therein. The light irradiation directed to the upper left or 10 oclock quadrantal area 41 of the screen is refracted in an improved manner by the chordal lens portion 95 to pull the light beam impingement radially toward the central axis 12. This refractive improvement photo-disposes the green dots 75 in that screen area in an improved equilateral triadical position with the contiguous red and blue dots 101 and 103, respectively. The maximum correction of dot placement is in the chordal effected area along the plane CD. Correction diminishes in a gradual manner on either side therefrom. The unmodified portion of the plano-concave lens effects light radiation refraction to provide substantially equilateral dot pattern exposure for the remaining area of the sensitized screen panel.

In photo-disposing the red dot pattern the optical system is positioned along plane EF which substantially corresponds to the 2-8 oclock panel diagonal, being removed clockwise from the 12 oclock position by 240 degrees. In FIG. while the whole optical system is shifted to coincide with plane EF only light source 69a is shown to avoid confusion in the drawing. In this position, the first chordal portion 95 effects refractive improvement for the red dot pattern in the upper right or 2 oclock quadrantal area 51 of the screen bringing the red dot positions 105 radially inward into an improved triadical position so that the subsequent electron beam impingement will be better centered thereon.

For blue dot deposition the optical system is oriented in the AB ordinate plane wherein light source 69b is indicated. With the optical system so positioned, the refractive improvement effected by the first chordal portion falls within an area 107 which is substantially outside of the rectangular panel and needs no correction, but in a round panel 106 correction of blue dot orientation would be desired in said area. Thus, in a rectangular screen the first chordal lens portion affords improved dot positioning for each of two color patterns in substantially a specific area of the screen for each. This facilitates a subsequent color display of improved color purity since the color dot patterns are more desirably positioned in these certain screen areas to be in better register with electron beam impingement. The various subsequent electron beam landings are designated in FIG. 5 to indicate the improvement produced.

Since the optical system 68 may be tilted With reference to the central axis 12 or the distance of offset of the lens or light source may be varied therefrom or the angle of lens tilt varied, it may be expedient to modify a second portion of the plano surface of the lens. A second embodiment of the invention as shown in FIG. 8 allows a change in the lens tilt-lens offset relationship wherein a second chordal portion 111 is modified to make correction to dot placement in the screen areas oriented substantially opposite the influence of the first chordal portion 95. The modified plano-concave lens 74, which has a usable refractive portion similar to that exhibited by refractive medium 71', as illustrated in cross-section in FIG. 8, has a first modified chordal portion with a substantially cylindrical surface 97 as previously described for the initial embodiment. In addition, there is a second modified chordal portion 111 substantially diametrically opposed to the first chordal portion 95 with an intervening substantially unmodified plano portion 93 therebetween; the second chordal portion being oriented substantially opposite the non-symmetrical spherical concavity 89. This section chordal portion, substantially involving the section v, is formed as a second substantially cylindrical surface 113 transitionally tangent to the plano surface 93 at a second substantially linear region of demarcation 108 to impart a slightly opposed concave curvature in a direction away from said spherical concavity and effect a gradual increase in the refractive thickness of the light refractive medium 74 in a second selected peripheral area thereof. The axis 109 of the second substantially cylindrical section is spacedly oriented from the plano surface 93 in a plane 115 substantially parallel thereto, the axis 109 being substantially parallel to the linear region of demarcation 108 and in a plane 119 therewith perpendicular to the plano surface 93. The radius 117 of the second substantially cylindrical surface, as determined from axis 109, is of a value to effect the required arcuate surface for consummating the desired refraction. The plane of symmetry of this refractive medium bisects both chordal portions and is oriented in the optical system as for the previously described embodiment. The utilization of this second light refractive medium facilitates dot placement compensation in diagonally opposite areas of the screen when such is desired.

While the modified first and second chordal portions of the plano lens surface have been described as being substantially circular cylindrical sections, optical requirements may necessitate forming the chordal surface in accordance with sequentially differing radii 94 and/ or 117 in effecting peripheral contours other than circular. If such be the case for either or both chordal surfaces, the planes and/or containing the respective axes 98 and 109 may be parallelly shifted in spaced relationship to the plano lens surface according to radial requirements. In addition, it may be desirable for optical considerations to effectuate a departure from cylindricity in all or a portion of each or both of the modified chordal surfaces. For example, with reference to FIGS. 7, 9, 10 and 11, the profile substantially cylindrical general surface 97 of the first chordal portion 95, as transversely defined by lines of intersection between planes parallel with the perpendicular plane 102 and the general chordal surface 97, may be a substantially straight surface as indicated by the substantially straight profile 121 as shown in the embodiment illustrated in FIG.

9. Optical requirements may necessitate the formation of a slightly curved surface, as for example, an arcuately ellipsoidal surface as characterized by the elliptic line 123 in the embodiment shown in FIG. 10, or an arcuately hyperboloidal surface as depicted by the hyperbolic line 125 in the embodiment illustrated in FIG. 11, or the profile surface may be a blended combination of the aforementioned surface embodiments. Similarly, with reference to FIGS. 8, 12, 13 and 14, the profile of the substantially cylindrical general surface 113 of the second chordal portion 111 as transversely defined by lines of intersection between planes parallel with the perpendicular plane 119 and the general chordal surface 113 may be a substantially straight surface as indicated by the substantially straight profile 127 as illustrated in the embodiment shown in FIG. 12. As with the first chordal profile, a slightly curved surface may be optically advantageous for the second chordal portion, such as that portrayed in the embodiment in FIG. 13, wherein an arcuately ellipsoidal surface is indicated by the elliptic line 129, or an arcuately hyperboloidal surface as shown in the embodiment of FIG. 14 by the hyper-bolic line 131, or a blended combination of the several surface embodiments. Thus, the term substantially cylindrica as used in this specification with reference to either or both of the modified chordal lens portions is intended to encompass peripheral surface contours transitionally ranging from substantially right cylindrical to cylindroidal manifestations and transverse profiles ranging from substantially straight to arcuate or respective combinations thereof.

Use of the improved method and associated optical system for patterned screen exposure has markedly improved color screen quality and has provided for the desired register of the dot patterns with electron beam impingement in tubes having deflections in excess of 70 degrees. Thus, a patterned cathodoluminescent screen is produced wherein the phosphor dots in substantially all portions of the screen are in contiguous abutment. This disposure arrangement forms an expansive substantially regular tessellation of dots which are discretely oriented as a multiplicity of similar triadical phosphor groupings. A screen so formed produces a vastly improved display having significantly enhanced color purity.

While there have been shown and described what are at present considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.

What is claimed is:

1. An optical system for use in a light exposure device utilized in the fabrication of a patterned cathodoluminescent screen for a color cathode ray tube having a central axis and at least one electron gun positionally related thereto from which emanates an electron beam capable of being deflected from a static path to scan said patterned screen through an intermediately positioned grid mask spaced adjacent to said screen, said optical system being positioned relative to said static beam path to discretely beam light through said mask to photo-dispose said screen pattern on a related screen-bearing panel, said system comprising:

a point source of light radiation having an axis and being directionally oriented relative to said masked coated panel to provide processing illumination thereto;

a modified plano-concave lens having a plane of symmetry with a quasi-axis of symmetry therein and a lateral axis of placement perpendicularly intersecting said quasi-axis and said plane of symmetry, said lens having the concave surface formed as a spherical concavity whereof the portion substantially utilized is a section eccentrically removed from but containing said quasi-axis defining a non-symmetrical spheri cal concavity, said plano surface being modified in at least one substantially chordal area thereof to provide improved light exposure for at least one specific peripheral quadrantal portion of said screen, sa1d lens being predeterminately positioned intermediate said light source and said masked panel with said concave surface being oriented toward said masked panel and said modified plano surface being oriented toward said point light source to provide non-symmeltrical optical aberration for said light radiation; an

means for predeterminately positioning said light source and said lens relative to said masked panel to provide a locus of motion of apparent light beam origin that is substantially coincident with the locus of motion of the apparent center of said electron beam deflection.

2. An optical system according to claim 1 wherein said modified plano surface has a first modified portion substantially opposite said non-symmetrical spherical concavity portion to form a first substantially chordal portion, said first chordal portion being shaped as a first substantially cylindrical section with the surface thereof tangent to said plano surface at a first substantially linear region of demarcation to impart a slight convex curvature to said modified surface in a direction toward said spherical concavity and effect a gradual reduction in the refractive thickness of said lens in a first selected substantially peripheral area thereof.

3. An optical system according to claim 2 wherein said lens is positioned in a manner that said plane of symmetry of said lens bisects said first chordal portion in a manner perpendicular to said substantially cylindrical section and has said quasi-lens axis, said light source axis, and said central axis contained therein.

4. An optical system according to claim 2 wherein said substantially cylindrical section of said first chordal portion of said lens has an axis spacedly oriented from said concave surface in a plane substantially parallel to said plano lens surface, said axis of said substantially cylindrical section being substantially parallel to said first substantially linear area of demarcation and in a plane perpendicular to said lens plane of symmetry.

5. An optical system according to claim 4 wherein said axis of siad first substantially cylindrical section is substantially parallel to said first substantially linear area of demarcation and in plane therewith perpendicular to said plano surface.

6. An optical system according to claim 2 wherein said modified plano surface of said lens has a second modified portion oriented substantially opposite said non-symmetrical spherical concavity to form a second chordal portion bemg substantially diametrically opposed to said first chordal portion, said second chordal portion being shaped as 7. An optical system according to claim 6 wherein said lens is positioned in a manner that the plane of symmetry of said lens bisects said first and second chordal portions in a manner perpendicular to said substantially cylindrical sections and has said quasi-lens axis, said light source axis, and said central axis coincident therewith.

8. An optical system according to claim 6 wherein said substantially cylindrical section of said second chordal portion of said lens has an axis spacedly oriented from said plano surface in a plane substantially parallel thereto, said axis of said second substantially cylindrical section being substantially parallel to said second substantially linear region of demarcation and in a plane perpendicular to said lens plane of symmetry.

9. An optical system according to claim 8 wherein said axis of said second substantially cylindrical section is substantially parallel to said second linear region of demarcation and in a plane therewith perpendicular to said plano surface.

10. An optical system according to claim 3 wherein said optical system is supported at a position offset from said central axis.

11. An optical system according to claim 3 wherein said optical system is supported at a position tilted from said central axis.

12. An optical system according to claim 3 wherein said optical system is supported at a position offset and tilted from said central axis.

13. An optical system according to claim 3 wherein said optical system is supported with said lens offset from said central axis.

14. An optical system according to claim 3 wherein said optical system is supported with said lens offset from said 12 central axis and tilted with. reference to said lateral axis of placement.

References Cited UNITED STATES PATENTS 2,817,276 12/1957 Epstein et a1. 951 3,380,354- 4/ 1968 Thornton 951 3,386,354 6/1968 Schwartz 95-1 3,420,150 1/ 1969 Kaplan 951 NORTON ANSHER, Primary Examiner R. M. SHEER, Assistant Examiner 

